This invention relates to metal alloys and more particularly to nickel-titanium base metal alloys which have shape change
The Nitinol alloys are nickel-titanium-base metal alloys having shape change memories. The general method for using the memory properties of these alloys is to:
(1) shape the alloy into a permanent form at a temperature below the temperature transition range (TTR);
(2) constrain the alloy in this shape;
(3) anneal the alloy at 500.degree. C.;
(4) cool the alloy to a temperature below the TTR;
(5) remove the constraint; and
(6) shape the alloy into an another form.
The alloy can then be converted from its other shape to its permanent shape by heating it to a temperature above the TTR. An excellent discussion about the theories and properties of Nitinol is given by William J. Buehler and William B. Cross, "55-Nitinol: Unique Wire Alloy with a Memory," Wire Journal, June 1969. Methods of preparing Nitinol are disclosed in U.S. Pat. No. 3,174,851, entitled "Nickel-Base Alloys," which issued to Buehler and Wiley on Mar. 23, 1965. The shape change memory properties of nickel-titanium alloys containing from 53.5 to 56.5 weight percent nickel, the remainder being titanium, are disclosed in U.S. Pat. No. 3,403,238 entitled "Conversion of Heat Energy to Mechanical Energy," which issued to William J. Buehler and David M. Goldstein on Sept. 24, 1968.
In the prior art, the usual method of changing the TTR was to change the ratio of nickel to titanium or to substitute cobalt or iron for nickel. A limitation of this previous method of alloying, has been the requirement to prepare by melting a different composition of alloy for each different transition temperature desired. This limitation presents significant economic disadvantages to the manufacturer of these alloys. In addition to an infinite number of TTR possibilities, it is difficult to precision alloy to control to a pre-selected composition. For example, a shift in total cobalt on the order of 0.2% of the total composition can change the midpoint (50% recovery) of the TTR by 8.degree. C., an unacceptable amount in many applications. Even worse from the standpoint of reproducibility, a shift of 0.2 weight percent nickel can shift the midpoint of the TTR by 25.degree. C.
Hence the alloy manufacturer may find it necessary to remanufacture the alloy or to prepare several melts of slightly different compositions to achieve his intended final composition. Normal melting losses make it exceedingly difficult to anticipate the final composition with adequate precision. The alloy manufacturer can encounter high scrap losses.
U.S. Pat. No. 4,144,057, entitled "Shape Memory Alloys," issued on Mar. 13, 1979, to Keith Melton and Olivier Mercier, discloses Nickel--the use of from 0.5 to 30 weight percent of copper and from 0.01 to 5 weight percent of at least one element selected from the group consisting of aluminum zirconium, cobalt, chrome, and iron in nickel-titanium alloys. They report that the transition temperatures in these alloys are less sensitive to compositional changes. The use of copper, however, is not desirable in some cases. Therefore, it is desirable to have another method of adjusting the TTR. Moreover, even when copper is used, it is desirable to have means of further fine tuning the TTR.